1. Field of the Invention
The present invention relates to a two-wavelength antireflection film, for preventing reflection of high-power light of two different wavelengths, such as a fundamental beam and a 2nd harmonic beam of various lasers.
2. Related Background Art
As a two-wavelength antireflection film for preventing reflection of high-power light of two different wavelengths, such as a fundamental beam and a 2nd harmonic beam of various lasers, there are already known, for example, a two-wavelength antireflection film consisting of a three-layered film of a .lambda./4 film of a refractive index of 1.82, a .lambda./4 film of a refractive index of 1.585 and a .lambda./4 film of a refractive index of 1.38 (.lambda.=707 nm) formed on a substrate of a refractive index of 1.52, for preventing the reflection of the fundamental beam (wavelength 1064 nm) and the 2nd harmonic beam (wavelength 532 nm) of a Nd:YAG laser (cf. THIN-FILM OPTICAL FILTERS 2nd Edition; H. A. Macleod, Macmillan Publishing Company, NY, p. 126), a wavelength converting element KTP (KTiOPO.sub.4 crystal with a refractive index of 1.7 to 1.85 in the visible region) provided with a two-wavelength antireflective film for preventing the reflection of fundamental beam and 2nd harmonic beam of Nd:YAG laser (cf. Japanese Patent Laid-Open Application Nos. 2-247601 and 4-333834), and a combination of two multi-layered films capable of antireflection respectively for two different lights, with inclusion of a non-interference layer for avoiding any influence to the antireflective property of either film (cf. Japanese Patent Laid-Open Application Nos. 2-127601 and 1-167701). These employ a highly refractive material with a refractive index exceeding 1.7 such as Y.sub.2 O.sub.3 or TiO.sub.2 in a layer contained in three- to five-layered film, and such highly refractive material tends to be not sufficiently resistant for the high-power light such as laser beam, because of the generally high absorbance.
The above-mentioned conventional technologies show very low durability to high-power laser beam, because, as mentioned above, they employ a highly refractive material of a high absorbance in a part of the multi-layered film of three to five layers. Also, the production cost becomes high because of complication of the manufacturing process, in case the number of layers increases to as many as five, and particularly if the layers have respectively different optical thicknesses.